Steam systems are known whih have gravity-type steam accumulators wherein the pressure and temperature both drop as steam is taken out. Displacement-type reservoirs are also known wherein the pressure is maintained almost constant by reintroduction of water into it through a pump so that only the temperature inside the vessel drops.
With both of these systems there is a considerable temperature change within the main energy-storing accumulator or vessel. The obvious result of this temperature change is considerable thermal expansion and contraction so that only a relatively limited service life of the unit is obtainable.